Q&A for all webinars



LevelBasic
The First webinar in the year of education was held by Harold McNair: Introduction to Gas Chromatography. We decided to start with this really basic seminar because we anticipated a large demand on these basics. And we were right, over 900 attended the webinar. The live webinar has been recorded and can be viewed here.

"Introduction to GC" has also been recorded as a video presentation. Click here to go to the series of five, click here to watch a free taster.

During the webinar we received a large number of questions. Professor Harold McNair has answered them. You will find the questions and answers below:

Introduction to Gas Chromatography

     “Introduction to Gas Chromatography” Webinar
                 28 October 2009
            Question and Answers

2. With sample cleanup, I doubt that the quantitative aspects are guarantueed?

  If you add an internal standard to the sample matrix before clean-up you will normally get better quantitative results.

4. My lab uses hydrogen as a carrier gas and for GC-MS and GC-FID instead of helium. Considering the flammability of hydrogen, as well as the expense, are there advantages to using hydrogen that outweigh these issues?

I see no practical advantage in using H2 over He today.

6. What is the min. quantity (In Grams) for the Sample?

For capillary columns, typical sample concentrations range from nanograms up to 100 micrograms in a solvent. Typical injection volumns are 1 microliter.
 

8.  Is there any reason why Hydrogen gas cannot be used as carrier gas when using TCD, even we are not interested in analyzing Hydrogen??

Hydrogen can be used with the TCD.  It cannot be used to measure hydrogen or helium.
 

10. How can i calculate the LOD of GC-MS by using calibration series

First measure the noise  level.  Then make up a series of pure standards, maybe 10ppm, 1ppm, 0.1ppm etc.  When the peak area is 10X noise level, that is the LOQ, limit of quantization.  When the peak area is 3X noise level, that is the LOD, limit of detection.  That peak is present in the sample, but you cannot report any valid concentration. 
 

12.  What about  the column unions for capillary?

Yes, use a zero dead volume fitting, or  end to end fitting of identical I.D.s.
 

14.  What is the advantage of split injection system - Prof Deokate U.A.

As capillary columns demand a very small amount of sample injected rapidly, a split injections meets both demands.  It is also very simple, open a split vent.
 

16.  Can  sample be vaporized at sample point or in GC oven rather than hot injection port?

Sample can be vaporized in a heated inlet liner, or inside the column itself (on-column injector or PTV).
 

18.  What would you recommend to replace glass wool in the SSL liner due to possible adverse activity when dealing with dirty samples?

Be sure to use “silanized” glass wool, and replace it frequently.
 

20.  Hello, about the sampling of very volatile samples especially iso-pentane. we introduce the sample as liquid but we loose the sample at the time of injection. we got very poor peaks. thank you

Use a gas sample valve; add a restrictor to the exit end of the valve to keep the gas sample loop  under high pressure, at least 2-4psi.This should keep the pentane in the liquid state.
 

22.  Can you touch upon the difference between a PTV injection port and the S-SL injection port?

Sorry, this question is beyond my expertise.  Chromedia will ask within their expert team.
 

24.  Which gives better resolution, thick or thin film?

Thin films give better resolution than thick films.  Please refer to the Golay Equation for capillary columns.
 

26.  I am carrying out a chemical reaction in solution  (with dry organic solvent 20 mL) in a flask with inert atmosphere. I suspect that the reaction releases molecular hydrogen gas, however bubbles are not seen. I think that H2 (about 0.1 mmol) is dissolved in the solvent. Could GC technique detect this H2?

H2 can easily be detected by GC, but not with a FID.  You will need to use a TCD or MS.
 

28.  Do you need to split injections with a megabore column?

Some injections can be made without splitting on a thick film, megabore column, but you need to start at a low temperature and then temperature programs the column.  A split injection will always give better resolutionon all capillary columns .
 

30. How would you trouble shoot a signal value that increases during a temperature increase.  This looks like the baseline has shot up during the final temperature stage.

Several options here. Could be column bleeding. Please check the troubleshooting section on Chromedia.

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